1. Field of the Invention
The present invention relates to an electric motor using permanent magnets.
2. Description of the Related Art
FIG. 1 is a cross sectional view showing one example of a conventional electric motor using brushless-type permanent magnets. A plurality (6 in the drawing) of permanent magnets 51 are fastened by adhesives or the like to the surface of the outer circumference of a shaft member 53 which is rotatably supported at its both ends by bearing or the like so as to form a rotor. A plurality of ring-shaped high permeability material plates 52, having teeth spaced apart on the inner circumference surface thereof and gapped from the outer circumference surface of the rotor, are laminated so as to form a stator. The rotation of the rotor is controlled relative to the stator in accordance with a signal from a detector or the like arranged at one end of the shaft member 53.
In conventional electrical motors, sintered materials such as ferrite or rate earth elements are generally used as materials for permanent magnets. However, these materials are fragile, and when subjected to impact, the possibility of damaging these materials exists, especially during assembly and transportation. In order to solve the above-mentioned problems, the permanent magnets are protected by winding a glass fiber or a carbon fiber around the permanent magnets, and further by providing reinforcement thereon using a resin or the like. However, this requires an increase in the number of fabrication processes, and the materials utilized are too costly. Also, since the high permeability material plates are produced by punching, those portions of the high permeability material plates into which a rotor is inserted cannot be used again and are abandoned, thus wasting materials. If the shaft member is formed of a high permeability material, the magnetic flux from the magnets brings about a magneto-resistance when it passes through the shaft member. On the other hand, when the shaft member is formed of a non-high permeability material, the magnetic flux from the magnets brings about a magneto-resistance which has substantially the same magnetic permeability as air. In the former case, a leakage of flux occurs and travels axially in the shaft member, thereby magnetically producing a negative influence upon a detector or a loading device positioned at one end of the shaft. In the latter case, the magnetic force of the permanent magnets is decreased, thus resulting in a reduction in efficiency.